When a firearm is discharged, gases generated by combustion of an explosive mixture in the firearm chamber propel a projectile through the firearm barrel and out the muzzle. These propellant gases exit the muzzle in the wake of the projectile and mix with the ambient air. The exiting gases cause or contribute to unwanted effects including, muzzle flash, recoil/muzzle lift, and interference with projectile stability.
Muzzle flash results from contact of the propellant with air at the muzzle. The propellant gas mixture, containing traces of unburned powder, remains extremely hot at the end of the barrel. Oxygen in the surrounding air combines with the hot gas to enable combustion of the residual chemicals, resulting in a visible flash of light just beyond the end of the barrel. Muzzle flash is undesirable because, among other things, it gives away the location of a shooter at night or under other low ambient light conditions.
Recoil is the reactive force against the gun barrel applied by the moving bullet and propellant. A substantial component of this reactive force is created by the forward ejection of the propellant out the muzzle. The recoil force is typically applied at a point above the center of gravity of the firearm and this, combined with the torque reaction generated by the rapidly spinning projectile, tends to pull the muzzle upward and to the right upon firing.
Projectile stability is affected by the exiting propellant gas that passes and surrounds the projectile immediately beyond the muzzle. The velocity of the propellant is typically about twice the velocity of the projectile, so that at exit some propellant moves around and in front of the projectile. The propellant immediately slows down in the air, causing drag on the projectile. More significantly, in the case of a firearm with a rifled barrel, the propellant exerts a force that makes the spinning projectile wobble or “yaw”, thereby causing the projectile to take longer to stabilize and decreasing the accuracy of the firearm.